HighlightThis research determined the significant genetic and phenotypic relationships between seedling root traits and nitrogen use efficiency (NUE), and further identified five QTL clusters for improving NUE in maize.

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Root system architecture (RSA) is seldom considered as a selection criterion to improve yield in maize breeding, mainly because of the practical difficulties with their evaluation under field conditions. In the present study, phenotypic profiling of 187 advanced-backcross BC(4)F(3) maize lines (Ye478 × Wu312) was conducted at different developmental stages under field conditions at two locations (Dongbeiwang in 2007 and Shangzhuang in 2008) for five quantitative root traits. The aims were to (1) understand the genetic basis of root growth in the field; (2) investigate the contribution of root traits to grain yield (GY); and (3) detect QTLs controlling root traits at the seedling (I), silking (II) and maturation (III) stages. Axial root (AR)-related traits showed higher heritability than lateral root (LR)-related traits, which indicated stronger environmental effects on LR growth. Among the three developmental stages, root establishment at stage I showed the closest relationship with GY (r = 0.33-0.43, P < 0.001). Thirty QTLs for RSA were detected in the BC(4)F(3) population and only 13.3 % of the QTLs were detected at stage III. Most important QTLs for root traits were located on chromosome 6 near the locus umc1257 (bin 6.02-6.04) at stage I, and chromosome 10 near the locus umc2003 (bin 10.04) for number of AR across all three developmental stages. The regions of chromosome 7 near the locus bnlg339 (bin 7.03) and chromosome 1 near the locus bnlg1556 (bin 1.07) harbored QTLs for both GY- and LR-related traits at stages I and II, respectively. These results help to understand the genetic basis of root development under field conditions and their contribution to grain yield.

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Plant height (PH), ear height (EH) and the PH/EH ratio have a great effect on plant lodging in maize (Zea mays L.) under intensive cropping systems. To understand the genetic mechanisms controlling PH, EH and the PH/EH ratio in response to nitrogen (N) or phosphorus (P) supply, a set of 218 recombinant inbred lines (RILs) was used to evaluate PH, EH, PH/EH ratio and grain yield (GY) and grain yield components under low N, low P and normal N and P supply (NP). A total of 100 QTLs (quantitative trait loci) were detected for the traits investigated. Several QTLs were found to be associated with PH (bin 1.06/1.07) only, both PH and EH (umc1692 at bin 5.01/5.03; umc2313-umc1006 at bin 6.02) and the PH/EH ratio (bnlg1484-bnlg1866 at bin 1.03; umc1164-um1757 at bin 4.01) which was also associated with grain yield. N-or P-inducible QTLs for PH, EH and the PH/EH ratio were found in both years and are sensitive to weather conditions.

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Root system architecture (RSA) plays an important role in phosphorus (P) acquisition, but enhancing P use efficiency (PUE) in maize via genetic manipulation of RSA has not yet been reported. Here, using a maize recombinant inbred line (RIL) population, we investigated the genetic relationships between PUE and RSA, and developed P-efficient lines by selection of quantitative trait loci (QTLs) that coincide for both traits. In low-P (LP) fields, P uptake efficiency (PupE) was more closely correlated with PUE (r = 0.48-0.54), and RSA in hydroponics was significantly related to PupE (r = 0.25-0.30) but not to P utilization efficiency (PutE). QTL analysis detected a chromosome region where two QTLs for PUE, three for PupE and three for RSA were assigned into two QTL clusters, Cl-bin3.04a and Cl-bin3.04b. These QTLs had favorable effects from alleles derived from the large-rooted and high-PupE parent. Marker-assisted selection (MAS) identified nine advanced backcross-derived lines carrying Cl-bin3.04a or Cl-bin3.04b that displayed mean increases of 22%-26% in PUE in LP fields. Furthermore, a line L224 pyramiding Cl-bin3.04a and Cl-bin3.04b showed enhanced PupE, relying mainly on changes in root morphology, rather than root physiology, under both hydroponic and field conditions. These results highlight the physiological and genetic contributions of RSA to maize PupE, and provide a successful study case of developing P-efficient crops through QTL-based selection.

Maize (Zea mays L.) root morphology exhibits a high degree of phenotypic plasticity to nitrogen (N) deficiency, but the underlying genetic architecture remains to be investigated. Using an advanced BC 4 F 3 population, we investigated the root growth plasticity under two contrasted N levels and identified the quantitative trait loci (QTLs) with QTL-environment (Q Â E) interaction effects. Principal components analysis (PCA) on changesofroottraitstoNdeficiency(DLN-HN)showedthatroot length and biomass contributed for45.8%inthesamemagnitude and direction on the first PC, while root traits scattered highly on PC2 and PC3. Hierarchical cluster analysis on traits for DLN-HN further assigned the BC 4 F 3 lines into six groups, in which the special phenotypic responses to N deficiency was presented. These results revealed thecomplicated root plasticity of maizein response to N deficiency that can be caused by genotypeenvironment (G Â E) interactions. Furthermore, QTL mapping using a multi-environment analysis identified 35 QTLs for root traits. Nine of these QTLs exhibited significant Q Â E interaction effects. Taken together, our findings contribute to understanding the phenotypic and genotypic pattern of root plasticity to N deficiency, which will be useful for developing maize tolerance cultivars to N deficiency.Keywords: Genotype-environment interactions; nitrogen stress; quantitative trait locus; root morphology; root plasticity; Zea mays L. Citation: Li P, Zhuang Z, Cai H, Cheng S, Soomro AA, Liu Z, Gu R, Mi G, Yuan L, Chen F (2016) Use of genotype-environment interactions to elucidate the pattern of maize root plasticgqy to nitrogen deficiency.